1. Field of the Invention
The present invention relates to an image scanner for reading out an image photoelectrically from an original, and more particularly to an image scanner provided with a shading correction device.
2. Background Arts
As an image scanner for reading out an image photoelectrically from an original, a reflective scanner for a reflective original and a film scanner for a photographic film have been known. The image scanner comprises a light source for illuminating the original, a photoelectric conversion device that converts an optical image into electric signals, and an image forming lens for forming an optical image of the original on the photoelectric conversion device. As the light source, a halogen lamp may be cited as an exemplar. As the photoelectric conversion device, an image sensor, like CCD (charge-coupled device), has been widely used. In the reflective scanner, the light reflected from the original is projected as an optical image onto the image sensor through the image forming lens. In the film scanner, the light traveling through a picture frame of the photographic film is projected as an optical image onto the image sensor through the image forming lens.
As well known in the art, the quantity of light that travels through the image forming lens is not uniform but decreases with radial distance from an optical axis of the image forming lens, so the brightness decreases toward the edges of the image field, and thus the optical image formed through the lens tends to be darker in its marginal area than its central area. Moreover, the quantity of illumination light from the light source is not always uniform in a perpendicular plane to the optical axis, i.e. on the surface of the original. For these reasons, even if the original has an uniform brightness or an uniform transmittance density, the optical image formed on the image sensor does not have an uniform luminance. Such an evenness is called “shading”.
In order to obtain high-quality image, it is usual in the image scanner to process the image signals from the image sensor so as to eliminate the influence of shading. For instance, image signals are detected through the image sensor without placing any original or while placing a reference original having an uniform density, and the value of the image signal of each individual pixel, i.e. a luminance value, is compared to a predetermined reference value. Then, a correction coefficient is calculated for each individual pixel such that the image signals after being corrected with the correction coefficients for the respective pixels have the same value as the reference value. By correcting image signals actually obtained from an original image with these correction coefficients, the influence of shading is eliminated from the image signals. Hereinafter, such a correction process will be referred to as shading-correction.
On the other hand, there are different sizes of photographic picture frames and originals, while it is desirable to maintain the size of the optical image formed on the image sensor as constant as possible in view of the image processing conducted later on the image signals. In addition, in view of the resolving power of the image scanner, it is preferable to form the optical image in a maximum size readable through a photoreceptive surface of the image sensor. For these reasons, it is conventional in the image scanner that the magnification of the image forming lens is controlled in accordance with the size of the original such that the optical image is formed in the maximum size on the image sensor. It is also conventional to form an optical image from a limited portion of the original by adjusting the magnification of the image forming lens appropriately. This treatment is called “cropping”.
Because the size of the optical image formed on the image sensor varies with the magnification of the image forming lens, shading conditions of the optical image on the image sensor also varies with the magnification of the image forming lens. Therefore, it is desirable to set up different shading correction values for different lens magnification steps. However, where the image sensor has a great number of pixels, e.g. 3,200,000 pixels, a very large memory capacity is necessary for storing correction values determined for individual pixels with respect to individual magnification values available, e.g. eight magnification steps.
If the illumination light from the light source is well diffused and thus the luminance is well equalized, it is possible to reduce the requisite number of shading correction values. However, since the illuminance decreases as the light is more diffused, it is necessary to increase the light intensity of the light source in order to increase the degree of diffusion of the illumination light. Then, it becomes necessary to use a large size halogen lamp, which results in increasing the size and the manufacture cost of the image scanner.
For the sake of minimizing the image scanner and saving the power consumption, some image scanners use light emitting diodes (LED) as the light source. In that case, a large number of light emitting diodes are arranged in a matrix. Since the light intensity of the light source consisting of the LEDs is insufficient as compared to the halogen lamp, it is also necessary to correct the image signals with shading correction values which vary from pixel to pixel and also vary depending upon the magnification of the image forming lens. Accordingly, a large capacity memory for storing a large volume of shading correction data.